Method of treating pistons



,Patented May 12, 1936- 2,040,324 I UNITED STATES PATENT OFFICE METHOD IOF TREATING PISTONS George L. Moore, Cleveland, Ohio, asslgnor toAluminum Company of America, Pittsburgh, Pa., a corporation ofPennsylvania No Drawing. Application March 21, 1935,

' Serial No. 12,257

13 Claims. (01. 29-1565) Light metal pistons are usually made ofaluforged, or otherwise fabricated, are first subminum base alloys whichare either cast or forged jected to a thermal treatment designed toplace or otherwise worked into a piston blank or shape. the metal insuch condition that its hardness This piston blank is then cut ormachined into may be improved. This treatment consists in the finalpiston form of the shape and dimenheating the piston for from about 15minutes to 5 sions required. Also, since high hardness is a about 6hours'at a temperature of between 900 quality much desired in pistons,the piston is F. and 1020 F., followed by rapid cooling. If treated bysuch thermal methods as will induce the piston is fabricated by aworking process in the aluminum base alloy'from which the pissuch as byforging, extrusion, etc., it may even 10 ton is made, a substantialincrease in hardness. be die quenched; that is to say, if the piston 10The aluminum alloys of which pistons are usually is fabricated at anelevated temperature, it may made are those which are capable, inthermal be removed from the dies after working, and, treatment, ofproducing a high hardness. Such while still at such a temperature thatthe alloyalloys should also have a coeflicient of thermal ingconstituents are in solid solution in amount 16 expansion as nearlyequal as possible to that of in excess of the amount soluble at roomtemperthe usual ferrous metal cylinder in which the ature, it may berapidly cooled, thereby placing piston reciprocates, and it is likewisedesirable it in such physical condition that it is amenable that thealuminum alloy be one which machines to the further steps hereinafterdescribed. In with relative ease.' Certain specific aluminum thiscondition the piston may be readily ma- 20 ba ll y a used for such p p st n rly chined' to final dimension without surface scufiso all arecharacterized by their capability of reing, tool wear, or any of theother disadvantages ponding on thermal treatment in increase ofencountered heretofore in the machining of hard hardness. Some alloysare better' than others pistons of certain alloys. The piston is now inwith respect to the maximum hardness obtained condition to develop itsmaximum hardness and by such treatment, the coeihcient of thermal ex- Ihave found that this hardness may be developed 25 pansio a d ma h qua is. Consequent y by installing the piston in the motor and opersomealloys are preferable to others and while ating the motor for a fewhours. this invention will be described with reference to The advantagesof this series of steps are many. Certain of the preferred alloys t eates broadly Heretofore all thermal treatments were comto the treatmentof pistons made of aluminum pleted and the piston brought to maximumhard- 30 base alloys which develop increased hardnesson ness beforemachining. The high temperatures thermal treatment, such as those alloyscontainand rapid quenches involved in the solution ing magnesium orsilicon or mixtures of these treatment sometimes caused casting warpage.and ther all y n elements which impart to the The surface of the pistonwas occasionally scored alloy the capacity of increa ing hardn ss by thror otherwise disfigured in handling. At the lower 35 mal treatment.temperatures incident to bringing the piston up It has been the practicein the preparation of to hardness, most alloys are. subject to growth.al m u a l y p s for us n a m r t apsome more so than others. It wastherefore p y the h rm treatments y i h th de r thought necessary tocomplete the thermal treat- 40 final hardness is developed in the pistonprior to ments before machining and installing the piston 40 the machnin op T e piston is then main the motor. Machining the hard pistonsinchined and, afterw r p d n e o r. duced excessive tool wear and it'wasfrequently This method of procedure has been followed as necessary touse expensive tools and special techusual practice despite the fact thatthe thermal nique to maintain a production schedule.

treatments, precedin as they do the machining. Aluminum base alloys ofthe type disclosed in 45 d l p maximum hardness in the piston before U.S. Patent 1,799,837 have been found to be parit is machined, thus makingmachining difficult ticularly amenable to the series of steps disclosedparticularly in the case of certain alloys as hereabove. These alloyscontain from about '7 to inafter described. about 15 per cent ofsilicon, from about 0.2 to

The general object of this invention is to proabout 3 per cent ofmagnesium, from about 0.3 50 vide methods by which the disadvantages ofthe per cent to about 4 per cent of copper,- and from older processes ofpreparing pistons may be elimabout 0.5 per cent to about 7 per cent ofnickel, inated. with the usual impurities. The silicon, copper Inaccordance with the present invention the and magnesium are theconstituents which are aluminum alloy piston blanks af er being cast,mainly instrumental in illustrating the advantages of my invention byreason of their susceptibility to change of condition under thermaltreatments. It had heretofore been found that from the standpoint ofeconomic commercial production, it was practically impossible to machinethese pistons after the maximum hardness had been developed by asolution treatment followed by an aging treatment prior to machining. Acompromise theatment was frequently resorted to, that is to say, thepiston was given a low temperature aging treatment without priorsolution treatment, thereby achieving an intermediate degree of hardnesswhich rendered the machining operation commercially feasible but whichwas never capable of developing maximum hardness. By my process it isnow possible to put this or any other heat treatable alloy in acondition to develop its maximum hardness in the motor by first givingit the high temperature solution theatment, machining it while it isrelatively soft, say between and Brinell, and finally, without furtherthermal treatment, installing it in the motor. Several additionalspecific advantages may be mentioned. The artificial aging step,heretofore deemed necessary before installation of the piston in themotor, has now been eliminated thereby eifecting a substantial saving infurnaces, other materials, labor, and time. The period when the pistonis coming up to maximum hardness in the motor is the period when themotor is undergoing its breaking-in, or in other words, when it is beingoperated with greatest care. If the alloy happens to be such that lowtemperature thermal treatments subsequent to fabrication or solutiontreatment cause it to exhibit growth, the phenomenon will be compensatedfor by preliminary wearing in of piston and cylinder, thereby renderingunnecessary calculated tolerances which allow for piston growth in thecylinder. Pistons which are at maximum hardness when installed do notwear in as readily as those which are softer.

The article on which I have been able to exhibit the advantageousproperties of the invention to a superior extent is a piston cast inpermanent mold from an aluminum base alloy containing about 14 per centof silicon, 0.8 per cent of copper, 1.0 per cent of magnesium, and 2.5per cent of nickel, with possible additions of other constituents. Priorto the date of the present invention it had been the practice to merelyage this alloy at a relatively low temperature before machining, therebyattaining an intermediatede gree of hardness because if subjected to thecombination solution and aging treatment, the hardness increased to suchan extent that machining was accomplished only with diificulty. It isnow possible to develop the maximum hardness in the piston by firstheating it at a temperature between about 900 F. and 1020" F. for from15 minutes to 6 hours, thereafter machining it while its Brinellhardness is between about 85 and 110, and finally installing it in themotor Where its hardness increases to between about and Brinell in thefirst few hours tion.

When pistons are not subject to an unusual amount of growth, it ispossible to age them artificially after machining in the soft condition,and before installing them in the motor, but in the preferred embodimentof my invention I avoid the expense involved and time lost in this extrastep.

The steps outlined hereinabove are applicable to alloys not specificallydisclosed herein but of motor operaness under the influence of the heatof the motor in which it operates, the steps comprising fabricating saidpiston from an alloy amenable to the development of increased hardnessby thermal treatment, heating the piston at an elevated temperature,rapidly cooling the piston, machining the piston to the desireddimensions, and, without; further thermal treatment, installing saidpiston in said motor.

2. In the process of treating a motor piston whereby the piston developsits maximum hardness under the influence of the heat of the motor inwhich it operates, the steps comprising fabricating the piston from analloy amenable to softening by heating to a temperature in excess of 900F. and to hardening by heat treating at a relatively low temperature,heating said piston at a temperature in excess of 900 F., rapidlycooling said piston, machining said piston tot final dimensions, and,without further thermal treatment, installing said piston in said motor.

3. In the process of treating a motor piston whereby the piston developsits maximum hardness under the influence of the heat of the motor inwhich it operates, the steps comprising casting the piston in a chillmold, removing said piston from said mold at an elevated temperature,rapidly cooling said piston, machining said piston to final dimensions,and, without further thermal treatment, installing said piston in saidmotor.

4. In the process of treating a motor piston whereby the piston developsits maximum hardness under the influence of the heat of the mo- I tor inwhich it operates, the steps comprising fabricating the piston, causingthe said piston to attain a Brinell hardness of between about 80 and 110by quenching it from an elevated temperature, machining the piston tofinal dimensions while within said hardness range, and, withoutfurther'thermal treatment installing said piston in said motor.

5. In the process of treating a motor piston whereby the piston developsits maximum hardness under the influence of the heat of the motor inwhich it operates, the steps comprising fabricating the piston, causingthe said piston to attain a Brinell hardness of between 80 and 110 byquenching it from an elevated temperature, machining the piston to finaldimensions while within said hardness range, and thereafter installingsaid piston in said motor.

6. In the process of treating a motor piston whereby the piston developsits maximum hardness under the influence of the heat of the motor inwhich it operates, the steps comprising casting the piston in a chillmold, removing said piston from said mold at an elevated temperature,rapidly cooling said piston, machining said piston to final dimensionsat a Brinell hardness of between 80 and 110, and, without furtherthermal treatment, installing said piston in a motor.v

7. In the process of treating a motor piston whereby the piston developsits maximum hardness under the influence of the heat of the motor inwhich it operates, the steps comprising fabricating the piston betweendies, removing the piston from the dies at an elevated temperature,rapidly cooling the piston, machining said piston to final dimensions,and, without further thermal treatment, installing said piston in amotor.

8. In the process of treating a motor piston whereby the piston developsits maximum hardness under the influence of the heat of the motor inwhich it operates, the steps comprising fabricating the piston from analuminum base alloy containing from 7 to 15 per cent of silicon, from0.2 to 3.0 per cent of magnesium, from 0.5 to 7.0 per cent of nickel andfrom 0.3 to 4.0 per cent of copper, heating the piston at a temperatureof between 900 F. and 1020 F., rapidly cooling the piston, machining thepiston to final dimensions, and, without further thermal treatment,installing said piston in said motor.

9. In the process of treating a motor piston whereby the piston developsits maximum hard ness under the influence of the heat of the motor inwhich it operates, the steps comprising fabricating the piston from analuminum base alloy containing from 7 to 15 per cent of silicon, from0.2 to 3 per cent of magnesium, from 0.5 to 7.0 per cent of nickel, andfrom 0.3 to 4 per cent of copper, heating the piston at a temperature offrom 900 F. to 1020 F. for from 15 minutes to 6 hours, rapidly coolingthe piston, and thereafter installing said piston in said motor.

10. In the process of treating a motor, piston whereby the pistondevelops its maximum hardness under the influence of the heat of themotor in which it operates the steps comprising casting the piston in achill mold from an aluminum base alloy containing from 7 to 15 per centof silicon, from 0.2 to 3.0 per cent of magnesium, from 0.5 to 7.0 percent of nickel, and from 0.3 to 4.0 per cent of copper, removing saidpiston from said mold after solidification but at a temperature inexcess of 900 F., rapidly cooling said piston, machining said piston toflnal dimensions, and, without further thermal treatment, installingsaid piston in said motor.

11. In the process of treating amotor piston whereby the piston developsits maximum hardness under the influence of the heat of the motor inwhich it operates, the steps comprising fabricating the piston from analuminum base alloy containing about 14 per cent of silicon, about 1.0per cent of magnesium, about 2.5 per cent of nickel and about 0.8 percent of copper, heating the piston at a temperature of from 900 F. to1020 F. for-from 15 minutes to 6 hours, rapidly cooling said piston,machining said piston to finished dimensions, and, without furtherthermal treatment, installing said piston in said motor.

12. In the process of treating a motor piston whereby the pistondevelops its maximum hardness under the influence of the heat of themotor in which it operates the steps comprisingfabricating the pistonfrom an aluminum base alloy containing about 14 per cent 'of silicon,about 1.0 per cent of magnesium, about 2.5 per cent of nickel'andabout-0.8 per cent of copper, heating the piston at an elevatedtemperature and cooling therefrom so as to induce in the piston aBrinell hardness of from 80 to 110, machining the piston to finaldimensions while in said hardness range, and, without further thermaltreatment, installing said piston in said motor.

13. In the process of treating a motor piston whereby the pistondevelops its maximum hardness under the influence of the heat of themotor in which it operates, the steps comprising fabricating the pistonfrom an aluminum basealloy containing a substantial amount of silicon,rapidly cooling the piston from an elevated temperature, machining thepiston to final dimensions at a Brinell hardness not in excess of 110,and, without further thermal treatment, installing said piston in saidmotor.

GEORGE L. MOORE.

